What Is the “Grandmother Hypothesis”?

Research indicates that one reason some women live such long lives is to help care for their grandchildren. Newer research further indicates the pattern might differ depending on the grandmother-grandchild gender relationship. This collection of research falls under the “Grandmother Hypothesis”.

Evidence suggests paternal and maternal grandmothers have different incentives when investing in their granddaughters or grandsons, due to the difference in genetic relatedness.

Known as the “Grandmother Hypothesis”, it is hypothesized that women can increase their genetic contribution post menopause by forging relationships to their grandchildren.

One such study¹ finds evidence to support the theory, while another² says otherwise. A team led by Leslie Knapp, a biological anthropologist, discovered the grandmother’s effect could vary depending on X-chromosome relatedness.

Dr. Knapp and her team evaluated birth and death records of seven different populations across North America, Europe, Asia and Africa. Records from a number of different periods were reviewed, dating all the way back to the 17th century. They looked at child mortality rate within the first three years of life and found effects varied based on X-chromosome relatedness. This is not equivalent in boys and girls.

Maternal and Paternal Grandmothers, Their Grandsons and Granddaughters

More specifically, maternal grandmothers have just 25% X-chromosome relatedness with both sexes of grandchildren. However, paternal grandmothers are 50% X related to their granddaughters, and 0% X related to their grandsons.

This led researches to surmise that paternal and maternal grandmothers have different incentives when investing in the futures of their granddaughters and grandsons. It appeared as though the paternal grandmother’s presence in the 7 populations had a harmful effect on the grandsons, with her presence linked to an increase in mortality.

However, grandmothers’ presence had a beneficial effect on paternal granddaughters. This could explain why there are many inconsistencies in studies that were previously conducted. In those studies the grandchild’s sex was not factored in to the equation.

Biologists frequently use genetic relatedness between families to explain multiple stages of evolution. Those include kin investment, tribe formation, offspring recognition, altruism, and parenting strategies. This is how genetic relatedness between grandchildren and grandmothers came under consideration. The gender link has implications for the entire field of human evolution.

Additionally, research shows grandmothers frequently played a vital role during the hunter gatherer stage of evolution. Other female primates failed to live beyond their childbearing years, however in humans the lifespan doubles with a bit of grand mothering. Perhaps this is because grandmothers provided the foundation for learning new skills, pair bonding, and cooperation.

What do you think? Is there any truth to these studies from your personal perspective?

Share your stories about how important your grandma was to you in the comments below.

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